Supersonic diffusers have a variety of applications: wind tunnels, ground testing of high altitude rocket engines, engine inlet for supersonic aircraft, and any supersonic device that operates at a static pressure below ambient. One such device is the continuous wave (cw) supersonic chemical laser. While the present invention focuses on this last application, it may be applicable to other supersonic flows where a diffuser is required.
A supersonic flow, where the pressure is below ambient, occurs in the laser cavity of a cw chemical laser. CW chemical lasers involve a steady, supersonic, low-pressure gaseous flow inside the laser cavity. In fact, all high-performance cw chemical lasers operate supersonically with a laser cavity pressure of a few Torr. For example, a chemical oxygen-iodine laser (COIL) typically operates in the 1 to 10 Torr range, although pressures as high as 20 Torr may be possible. A pressure value above about 4 Torr usually is achieved by adding diluent gas to the singlet oxygen generator (SOG) that drives the COIL device. A diffuser is then used to increase the device's exit pressure. If this exit pressure is still below ambient, the diffuser is followed by a pumping system that typically consists of mechanical pumps or an ejector system. In specialized cases, chemical pumping may be used.
The type of high-performance laser under consideration might be mounted on a motorized vehicle, naval vessel, or on an aircraft. It requires a pressure recovery system to increase the pressure of the high-speed, spent laser effluent from its several Torr value to a pressure level slightly above ambient. At sea level, ambient pressure is about 760 Torr, while ambient pressure for an aircraft at a 40,000 foot altitude would be about 141 Torr. In any case, the size and weight of the pressure recovery system is of crucial importance for the viability of the overall laser system. It is thus important that the pressure recovery system be as compact and lightweight as possible for the intended application.
Diffusers are common devices whose function is to convert as much as possible of the kinetic energy of a fluid, at the inlet of the diffuser, into an increased value for the pressure of the fluid at the device exit. The exit pressure, Pr, is referred to as the recovered pressure. Diffusers come in two categories, depending on whether the inlet flow is subsonic or supersonic. Supersonic diffusers are bulky and generally very inefficient, especially when their inlet Mach number is large.
Diffusers used for chemical lasers generally consist of a converging supersonic section, followed by a throat region, which is then followed by a slowly diverging subsonic diffuser. The throat region and subsonic section can be lengthy. Most of the pressure increase occurs in the throat region, which is a duct containing a system of oblique shock waves. A subsonic diffuser, in order to avoid boundary-layer separation, increases its cross-sectional area gradually. This subsonic section only provides a modest amount of pressure recovery, typically less than 10% of the overall value of a supersonic/subsonic diffuser. For reasons of compactness, the subsonic portion of the diffuser may be attenuated or even bypassed.
An oblique shock system generally starts at the diffuser's inlet and continues into the throat section. Most of the static pressure increase stems from the shock system and not from an isentropic process. By contrast, the stagnation pressure steadily decreases through the shock system. The overall decrease in the stagnation pressure of a diffuser is of crucial importance. At the exit, where the Mach number is small compared to unity, the recovered (static) pressure is essentially the stagnation pressure. A diffuser's efficiency is, therefore, just the ratio of the exit to inlet stagnation pressures. At a supersonic inlet, the stagnation pressure significantly exceeds the static pressure. For steady, isentropic flow, the stagnation pressure is a constant, and the diffuser's second-law efficiency is thus 100%.
The standard pressure recovery system for any supersonic gas flow would be a supersonic/subsonic diffuser as is commonly used with a wind tunnel. Common to such diffusers is a poor efficiency because an oblique shock system is present inside the supersonic section, including the throat section, of the diffuser. The shock system decreases the stagnation pressure of the flow and causes boundary-layer separation.
Use of a conventional supersonic/subsonic diffuser for a chemical laser has a number of major drawbacks, aside from its poor efficiency. They are bulky, heavy, and for a conventional COIL do not provide nearly enough pressure recovery, even for aircraft operation of a COIL at a 40,000 foot altitude. For a conventional COIL system with diluent in the SOG and a diffuser, an ejector system is still required for an aircraft-based system. Compared to a diffuser, an ejector system may be more compact but is heavier, much heavier, as the laser run time increases.
Accordingly, there is a need for a supersonic diffuser able to provide greater pressure recovery than existing diffuser designs while remaining relatively compact and light in weight.